1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus which provides power assist of an electric motor directly to a steering system so as to reduce manual steering effort to be applied by the driver.
2. Description of the Related Art
FIG. 1 of the accompanying drawings diagrammatically shows the general construction of an electric power steering apparatus of the type concerned. The electric power steering apparatus 1 includes an electric motor 10 incorporated in a steering system, and a control unit 20 for controlling power assist supplied from the electric motor 10, so as to reduce the manual steering effort or force required by the driver.
The steering system includes a steering wheel 2 attached to an end of a steering shaft 3. The opposite end of the steering shaft 3 is connected to one end of a connecting shaft 4 via a first universal joint 4a, the other end of the connecting shaft 4 being connected via a second universal joint 4b to a pinion 6 of a rack-and-pinion mechanism 5. The pinion 6 is in mesh with a rack 7 which is a long bar with gear teeth 7a cut into one side. The rack-and-pinion mechanism 5 translates a rotary motion of the pinion 6 into an axial reciprocating motion of the rack 7. Opposite ends of the rack 7 are connected via tie rods 8 to steerable left and right front wheels 9, 9. When the steering wheel 2 is manually turned or rotated in a desired direction, the rack-and-pinion mechanism 5 and the tie rods 8 cause the front wheels 9 to pivot in the same direction to thereby change the direction of movement of a motor vehicle.
In order to reduce the manual steering effort or force required by the driver, the electric motor 10 is disposed in concentric relation to the rack 7 and supplies an assist torque (steering assist torque) to the rack 7 via a ball screw mechanism 11. The ball screw mechanism 11 converts rotational power of the electric motor 10 into an axial thrusting force acting on the rack 7. The ball screw mechanism 11 is generally comprised of a nut 12 connected to a rotor of the electric motor 10, and a threaded screw portion 7b formed along a longitudinal portion of the rack 7. By virtue of the threaded engagement between the nut 12 and the threaded screw portion 7b, a rotational force of the nut 12 is converted into an axial thrusting force of the rack 7. Thus, the assist torque generated by the electric motor 10 is translated into the axial thrusting force of the rack 7 by which manual steering effort required by the driver to turn the steering wheel 2 is reduced.
A steering torque detecting section (steering torque sensor) 18 detects a manual steering torque Ts acting on the pinion 6 and supplies a torque signal Tp indicative of the detected steering torque Ts to the control unit 20. The control unit 20 outputs, on the basis of the torque signal Tp, a motor control signal 20a to control output power (steering assist torque) of the electric motor 10.
FIG. 2 of the accompanying drawings shows in block diagram the general arrangement of a conventional control unit. The control unit 20A includes a target assist torque determining section 201 and a motor drive section 202. The target assist torque determining section 201 determines a target assist torque on the basis of the torque signal Tp and outputs the determined target assist torque in the form of a target assist torque signal 201a. More specifically, the target assist torque determining section 201 sets the target assist torque to be zero when an absolute value of the steering torque is less than a predetermined dead zone threshold. Conversely, when the absolute value of steering torque is greater than the predetermined dead zone threshold, a target assist torque which is proportional to the steering torque is output from the target assist torque determining section 201. The target assist torque output from the target assist torque determining section 201 is limited below an upper limit even when the steering torque increases excessively.
The motor drive section 202 determines an offset between the target assist torque signal 201a supplied from the target assist torque determining section 201 and a motor current signal IM supplied from a current detector (not shown) provided to detect a current actually flowing in the electric motor 10, and generates a motor drive signal 20a in such a manner as to render the offset zero. The motor drive signal 20a is supplied to the electric motor 10 with the result that the target assist torque is supplied from the electric motor 10.
FIG. 3 shows in block diagram the general arrangement of another conventional control unit. The control unit 20B includes a first target assist torque determining section 211, a steering torque differentiating section 212, a second target assist torque determining section 213, an adding section or adder 214, and a motor drive section 202.
The first target assist torque determining section 211 determines a first target assist torque on the basis of the torque signal Tp and outputs the determined first target assist torque in the form of a first target assist torque signal 211a. More specifically, the first target assist torque determining section 211 sets the first target assist torque to be zero when an absolute value of the steering torque is less than a predetermined dead zone threshold. Conversely, when the absolute value of steering torque is greater than the predetermined dead zone threshold and less than a predetermined threshold, a first target assist torque which is proportional, with low gain, to the steering torque is output from the first target assist torque determining section 211. A steering torque greater than the predetermined threshold causes the first target assist torque determining section 211 to output a first target assist torque which is proportional, with high gain, to the steering torque. The first target assist torque output from the first target assist torque determining section 211 is limited below an upper limit even when the steering torque increases excessively.
The steering torque differentiating section 212 determines a variation per unit time of the torque signal Tp and outputs the determined variation in the form of a differential torque signal 212a (Tp.multidot.s in a Laplace transform range).
The second target assist torque determining section 213 determines a second target assist torque on the basis of the differential torque signal 212a and outputs the determined second target assist torque in the form of a second target assist torque signal 213a. The second target assist torque output from the second target assist torque determining section 213 is limited below an upper limit even when the differential torque value becomes excessively large.
The adder 214 adds together a signal 211a corresponding to the first target assist torque and a signal 213a corresponding to the second target assist torque and outputs the result of arithmetic operation (addition) in the form of a target assist torque signal 214a.
The motor drive section 202 determines an offset between the target assist torque signal 201a supplied from the target assist torque determining section 201 and a motor current signal IM supplied from a current detector (not shown) provided to detect a current actually flowing in the electric motor 10, and generates a motor drive signal 20a in such a manner as to render the offset zero. The motor drive signal 20a is supplied to the electric motor 10 with the result that the target assist torque is supplied from the electric motor 10.
The conventional control unit 20A shown in FIG. 2 seeks to improve the response characteristic by adjusting gain of the gain of a steering power assist system in which the input is the steering torque signal Tp and the output is the motor drive signal 20a or the steering assist torque supplied from the electric motor 10. The conventional control unit shown in FIG. 3 seeks to enhance the response characteristic by achieving adjustment of the gain of the steering power assist system in response to variations of the steering torque. The conventional control units, however, have a drawback that an excessively high gain of the steering power assist system would harm the operational stability of the steering power assist system. Another drawback with the conventional control units is that due to the influence of an inertial efficiency of the electric motor and a friction produced in a speed reducing mechanism, a sufficiently high level of response cannot be achieved. The conventional electric power steering apparatus as a whole cannot perform steering operation with steering feeling (response characteristic) at least comparable to that of the manual steering apparatus.